Can you please be more specific, in particular which material was chosen for the
print ?
My guess is that it was one of the dyed strong+flexible options - there is a
tutorial in the post-processing forum that describes the process of dyeing
the white-strong-and-flexible material that mentions just this kind of issue.
Powdery material trapped within the hollow model will pick up water during
the dyeing and it may take several weeks or even months until the moisture
(and stain) is gone. Try to make the opening of your model a bit larger if you
can, or add a second opening - if I understand the production process correctly,
the powder is removed by compressed air.
EDIT: actually
this thread in the
"it arrived" forum is where i picked up the reference to the document (by shapeways user "gibell")